1. Technical Field
This disclosure relates to a heat-sensitive pressure-sensitive adhesive that is non-adhesive at room temperature, however, can exhibit surface tackiness and maintain the surface tackiness even after the surface tackiness is exhibited, and a heat-sensitive adhesive material using the heat-sensitive pressure-sensitive adhesive.
2. Description of the Related Art
Recently, label adhesive materials are increasingly used for price labels, product (bar code) labels, quality labels, ingredient labels, and advertisement labels (stickers). As for label recording methods, various methods are developed such as inkjet recording methods, heat-sensitive recording methods, and pressure-sensitive recording methods.
Those label adhesive materials typically have a structure, for example, in which a tacky layer and a release paper are laminated on the opposite surface from a surface of the label on which information is recorded. Such a label adhesive material is widely used because after the release paper is peeled off, and the label adhesive material can be easily affixed by only pressure. Generally, after a release paper is peeled off from a label adhesive material, and a label is used, however, it is difficult to collect and recycle the peeled release paper, and in most cases, the peeled release paper discarded subsequently.
Then, recently, a heat-sensitive adhesive label sheet having a heat-sensitive tacky layer containing a heat-sensitive pressure-sensitive adhesive that does not exhibit surface tackiness at room temperature but can exhibit surface tackiness by application of heat and requires no release paper has been a focus of attention (see Japanese Utility Model Application Laid-Open (JP-U) No. 6-25869).
The heat-sensitive adhesive material used for the heat-sensitive adhesive label sheet, as described in Secchaku Binran “Adhesive Handbook”, 12th Ed., Kobunshi Kankokai (1980), on pp. 131-135 etc., basically contains a thermoplastic resin having a relatively high glass transition temperature of 0° C. or more and a low-molecule weight compound and further contains a tackiness imparting agent in accordance with necessity.
However, after the heat-sensitive adhesive material used for the heat-sensitive adhesive label sheet exhibit surface tackiness, the adhesive strength degrades with a lapse of time. Further, there is a need to apply a high thermal energy when thermally activated.
To solve the problem, a technique is proposed to reduce thermal energy i.e., impart high sensitivity when thermally activating such a heat-sensitive adhesive label sheet by forming a heat-insulating layer containing a plastic hollow particle and a water-soluble binder in between a support and a heat-sensitive tacky layer (see Japanese Patent (JP-B) No. 2683733 and Japanese Patent Application Laid-Open (JP-A) No. 10-152660).
These proposed heat-sensitive adhesive label sheets are effective in reducing thermal energy when the heat-sensitive tacky layer is thermally activated, however, the adhesive strength to rough-surfaced adherends such as corrugated board is short of the practical level because the water-soluble binder that does not exhibit surface tackiness at room temperature is used therein. In addition, these proposed heat-sensitive adhesive label sheets have not yet resolved the problem that the adhesive strength after exhibiting the surface tackiness degrades with a lapse of time.
As just mentioned above, a heat-sensitive tacky layer in a heat-sensitive adhesive label sheet has shortcomings that the adhesive strength after exhibiting its surface tackiness degrades with time and the adhesive strength to rough-surfaced adherends such as corrugated board is weak. Further, such a heat-sensitive tacky layer in a heat-sensitive adhesive label sheet further has a problem that the surface tackiness is inconveniently exhibited when stored in a roll shape, and a heat-sensitive tacky layer sticks to the back surface of the label sheet (blocking).
There is a contradictory relation between adhesive strength to adherends and blocking resistance. In other words, when adhesive strength is improved, blocking resistance is degraded. In contrast, blocking resistance is improved, adhesive strength to adherends is reduced. Thus, it is very difficult to improve both of the properties, and the present situation is that it has not yet achieved.
For the purpose to enhance the adhesive strength, for examples, attempts to improve adhesion property of label sheets by using ester materials such as dicyclohexyl phthalate as a thermofusible material have been taken so far (see Japanese patent Application Laid-Open (JP-A) Nos. 61-9479 and 7-278521).
However, there is a tendency that adhesive properties of label sheets to adherends are improved by using these thermofusible materials, however, it is still impossible to maintain a stable adhesive strength of a label sheet to rough-surfaced adherends such as corrugated board. Further, when such a label sheet is stored in a roll shape, the label sheet is likely to stick to the back surface thereof, and blocking resistance thereof is significantly low.
In the meanwhile, as a heat-sensitive adhesive material and a heat-sensitive pressure-sensitive adhesive to improve blocking resistance, which is another problem in this area, conventional ones containing various filler components are already known in the art. As additives usually used in a heat-sensitive tacky layer, blocking inhibitors such as inorganic particles and organic particles are also disclosed in the disclosed proposals stated above.
As examples actually using the method, there have been proposed, for example, a method in which a heat-sensitive tacky layer and a thermoplastic resin layer are formed on a support, and a pigment component is contained in the thermoplastic resin layer (see Japanese Patent Application Laid-Open (JP-A) No. 11-279495), and a method in which an oil-absorbing pigment (oil absorption: 100 mL/100 g) is contained in a heat-sensitive tacky layer containing a thermoplastic resin and a solid plasticizer as its main components (see Japanese Patent Application Laid-Open (JP-A) No. 9-235528).
However, any of these proposals could cause a reduction in adhesive strength to adherends because pigment components inhibiting the adhesive strength is added to the outermost layer and cannot improve blocking resistance and adhesive strength. Therefore, they are not suitable for practical use.
Generally, a thermoplastic resin contained in a heat-sensitive pressure-sensitive adhesive is used to give surface tackiness and adhesive force, and a thermofusible material is used to fluidize or soften a thermoplastic resin which is a solid at room temperature (at 20° C.) and has a relatively high glass transition temperature of 0° C. or more, and an adhesion imparting agent to be further added as needed has a function to improve surface tackiness.
In exhibition of the adhesive strength when such a heat-sensitive pressure-sensitive adhesive is used, the adhesiveness of heat-sensitive pressure-sensitive adhesive to the surface of an adherend to be affixed is important, and when irregularities or convexoconcaves of the adherend surface are large, it is hard for the heat-sensitive pressure-sensitive agent to sufficiently exhibit its function. Generally, to exhibit its surface tackiness to an adherent surface having large irregularities or convexoconcaves (rough surface), in heat-sensitive pressure-sensitive adhesives including typical pressure-sensitive adhesives, it is effective to thicken the thickness of a heat-sensitive tacky layer formed in a heat-sensitive pressure-sensitive adhesive, and the thickened tacky layer can compensate for the irregularities of the adherend surface.
However, to thicken the thickness of a heat-sensitive tacky layer is inefficient and is not a practical method because the method is disadvantageous in production cost and that the method requires significantly large amount of thermal energy when the whole of the heat-sensitive tacky layer is heated.
For another method to compensate for irregularities of an adherend surface, softening a heat-sensitive pressure-sensitive adhesive can be considered. In this case, it can be considered that flexibility of the heat-sensitive pressure-sensitive adhesive is changed by a thermoplastic resin and a thermofusible material contained in materials constituting the heat-sensitive pressure-sensitive adhesive. Aiming to improve surface tackiness of a heat-sensitive pressure-sensitive adhesive to not only rough-surfaced adherends but also various adherends, a variety of techniques are proposed.
For example, Japanese Patent Application Laid-Open (JP-A) Nos. 6-57226 and 6-57233 respective propose to use an ethylene-vinyl acetate copolymer having a glass transition temperature of 0° C. or more or a thermoplastic resin (excluding ethylene-vinyl acetate copolymers) having a glass transition temperature of −5° C. or higher, etc as a thermoplastic resin. However, the adhesive strength of the heat-sensitive pressure-sensitive adhesive to vinyl chloride wrap and polyolefin wrap has not yet reached the practical level, although it can obtain relatively favorable results of the adhesion to stainless steel plates.
Further, there are proposals which incorporates improved points to layer structures and heat-fusible materials. For example, Japanese Patent Application Laid-Open (JP-A) No. 9-265260 proposes a heat-sensitive adhesive material that has a base, a heat-sensitive chromogenic layer, and an under layer containing a non-foamable hollow particle formed in between the base and the heat-sensitive chromogenic layer, in which a heat-sensitive pressure-sensitive adhesive containing dicyclohexyl phthalate as a thermofusible material. The heat-sensitive adhesive material achieved a generally satisfactory level in improving heat-sensitivity of the heat-sensitive chromogenic layer and preventing color developing of the ground of the heat-sensitive chromogenic layer when thermally activated, however, when the heat-sensitive adhesive material is affixed, blocking phenomenon occurs at around 40° C., i.e., an unintended adhesive function is exhibited, therefore, the proposed heat-sensitive adhesive material have not yet reached the level needed for practical use.
Further, Japanese Patent Application Laid-Open (JP-A) Nos. 2003-206455 and 2002-38123 respectively propose a heat-sensitive pressure-sensitive adhesive (a delayed tack adhesive) using benzophenone as a thermofusible material. These heat-sensitive pressure-sensitive adhesives respectively have a favorable adhesive strength to mirror finished surfaces like polyolefin and glass, however, have a weak adhesive strength to rough-surfaced adherends such as corrugated boards. When the heat-sensitive pressure-sensitive adhesive is left for a long time after affixed to a corrugated board, the adhesive strength is reduced, resulting in a significant trouble when practically used in physical distribution such as door-to-door parcel delivery, and further, there is a problem that a blocking phenomenon occurs under the conditions of temperature 60° C.
Japanese Patent Application Laid-Open (JP-A) No. 2002-146303 proposes a technique of forming two or more layers of heat-sensitive tacky layer, in which the temperature at which the maximum adhesive strength of the respective heat-sensitive tacky layers can be exhibited differs to each other. According to the technique, the surface tackiness of the heat-sensitive pressure-sensitive adhesive can be exhibited in a wide temperature range of low temperature environments to high temperature environments, however, the adhesive strength to rough-surfaced adherends such as corrugated boards is weak, and a blocking phenomenon may occur due to the effect of a layer capable of exhibiting adhesion at a relatively low temperature.
Japanese Patent (JP-B) No. 3556414 proposes a delayed tack glue using benzotriazole as a thermofusible material. The proposed tack glue is relatively excellent in blocking resistance and can maintain a constant adhesive strength to adherends composed of materials such as paper, glass, metal; and polyolefin resins such as polypropylene and polyethylene for a long period of time, however, it has problems that it does not exhibit its adhesion function under low temperature environments, and the adhesive strength is reduced after affixed to a corrugated board and left for a long time, and the problem posed a major obstacle when practically used in physical distribution such as door-to-door parcel delivery.
Further, as materials that are excellent in adhesion property to adherends and further excellent in blocking resistance, a heat-sensitive pressure-sensitive material using a phosphorus compound, and various adhesive sheets using the heat-sensitive pressure-sensitive material and the like are proposed (see Japanese Patent Application Laid-Open (JP-A) Nos. 2000-103969, 2000-191920, 2000-191922, 2000-212527, 2001-64401, 2001-262117, 2002-88678, 2002-338935, and 2004-117941).
Of these, Japanese Patent Application Laid-Open (JP-A) Nos. 2000-103969, 2000-191920, and 2000-191922 respectively propose a method to improve the adhesive strength of heat-sensitive pressure-sensitive adhesives by using a phosphorous compound having a melting point of 55° C. to 100° C. as a thermofusible material. However, according to the proposals, a blocking phenomenon easily occur because the melting point of the heat-sensitive tacky layer is lowered from 40° C. to 60° C., and under high temperature environments of 40° C., the heat-sensitive tacky layer does not have stickiness and the adhesive strength becomes significantly weak.
JP-A No. 2004-117941 stated above proposes a method to improve both blocking resistance and adhesion function by using a plurality of thermofusible materials selected from phthalic acid esters, phosphorous compounds, phosphate esters, hindered phenol compounds and triazole compounds. However, according to the proposed method, the adhesive strength is reduced with the lapse of time depending on the used thermofusible material, and surface tackiness cannot be exhibited in a wide temperature range of low temperatures (0° C.) to high temperatures (40° C.). Thus, these problems posed a major obstacle in practical use.
Recently, the present inventors proposed in JP-A No. 2006-111865 a heat-sensitive adhesive label sheet having at least a tacky under layer containing a thermoplastic resin having a glass transition temperature (Tg) of −70° C. to 0° C. and a filler. With use of the heat-sensitive adhesive label sheet, a favorable adhesive strength to rough-surfaced adherends such as corrugated boards can be obtained under the environments of normal temperature and normal humidity (23° C., 65% RH) and even when heated with a thermal head. However, the heat-sensitive adhesive label sheet is insufficient in adhesive strength to rough-surface adherends such as corrugated boards under low temperature environments of 0° C. to 10° C., and when the added amount of a resin for the tacky under layer is increased to increase the adhesive strength under low temperature environments, there is a problem that its blocking resistance is degraded.
As described above, a number of studies on heat-sensitive pressure-sensitive adhesives and heat-sensitive adhesive materials centering on thermofusible materials are provided, however, the present situation is that achieving both improvements in adhesion function and blocking resistance is still at an unsatisfactory level because of its contradictory relation between adhesion function and blocking resistance, and a heat-sensitive pressure-sensitive adhesive and a heat-sensitive adhesive material that has comprehensively excellent properties have not yet been provided so far.